1. Field of the Invention
The present invention relates to an image heating apparatus of induction heating type adapted for use in an image forming apparatus such as a copying apparatus or a printer utilizing an electrophotographic recording technology or an electrostatic recording technology, and an excitation coil unit adapted for use in such apparatus.
2. Description of Related Art
In an image forming apparatus such as a copying apparatus, a printer or a facsimile apparatus utilizing for example an electrophotographic process, there is provided a heat fixing apparatus for thermally fusing a toner image, transferred onto a recording material such as a plain paper or an OHP sheet, thereby fixing such image onto the recording material.
The image forming apparatus utilizing such electrophotographic process is often continuously powered even when a printing operation is not executed. In the mode of use of ordinary users, a time during which the powered image forming apparatus is not used (namely stand-by time) is far longer than a time of use (printing time). During such stand-by time, a largest electric power is consumed by the fixing apparatus. Therefore, by reducing the electric power consumption of the fixing apparatus during the stand-by time, it is possible to suppress the total electric power consumption of the image forming apparatus, both in the print time and in the stand-by time. On the other hand, when the user executes a printing operation, it is desirable that the image forming apparatus can be started up from the stand-by state to the printable state within a time as short as possible. However, in case the power consumption of the fixing apparatus is suppressed during the stand-by time, the start-up time to the printable state becomes longer as the temperature of the fixing apparatus is already lowered.
As a countermeasure, it is being tried to employ an induction heating method in which a heating roller, constituted of a cylindrical conductive member, generates heat by itself, instead of a conventionally common method of heating the heating roller by a halogen lamp heater, thereby further reducing the start-up time of the heating roller to the fixing temperature and thus achieving an energy saving.
In such induction heating method, a high frequency current is given to an excitation coil and a generated magnetic flux is caused to act on a cylindrical conductive member, thereby generating an eddy current in the conductive member and heating the heating roller by a Joule's heat generated by such eddy current.
An example of such prior fixing apparatus is illustrated in FIGS. 8 and 9.
In a fixing apparatus of induction heating type shown in FIG. 8, a heating roller 50 is provided rotatably and is formed by a cylindrical conductive member. A pressure roller 51 is pressed under a predetermined pressure to the heating roller 50, and is rotated in a direction a together with the heating roller 50 to pinch and convey a recording medium 54, moving in a direction b in a nip portion formed between the heating roller 50 and the pressure roller 51, thereby applying a heat and a pressure to the recording medium and achieving an image fixation. A coil assembly 52 for generating a magnetic field, for generating an induction current in the heating roller 50, is supported by a holder 64, and constitutes a holder unit 53 as a whole.
The coil assembly 52 includes a core 63 of a magnetic material, a bobbin 65 having a through hole for inserting the core 63, and an excitation coil 62 formed by winding a copper wire around the bobbin. The bobbin 65 functions as an insulating part for insulating the core 63 and the excitation coil 62. FIG. 9 shows only the holder unit 53 provided in the heating roller.
However, the fixing apparatus utilizing such induction heating method has been associated with following drawbacks.
The excitation coil is not heated by radiation and conduction of heat from the heating roller which generates heat, but also generates heat by Joule's heat, because a current flows in the excitation coil itself. Therefore, in case a fixing apparatus of induction heating type is mounted on a high-speed apparatus with a higher print output per unit time, the coil temperature is gradually elevated in a prolonged operation and may eventually exceed a heat resistant temperature of the coil.
In particular, a resinous insulation layer provided on the wire of the coil may be fused to cause a shortcircuiting.
In order to avoid such situation, it is proposed to blow air in the interior of the excitation coil along an axial direction thereof, thereby suppressing an increase in the coil temperature.
On the other hand, there is also proposed to reduce the heat generation of the coil itself thereby causing a curve of the coil temperature to saturate at a lower temperature. More specifically, there is adopted a method of reducing a resistance of the coil wire by employing a larger diameter in the wire constituting the coil or by employing a litz (twisted) wire, formed by bundling plural fine wires, for forming the coil, or a method of forming the cylindrical conductive member with a material of a high electrothermal converting efficiency thereby obtaining a higher output with a smaller current.
However, in the method of suppressing the temperature increase of the coil by air blowing, among the electric power supplied to the coil of the fixing apparatus, a heat corresponding to the Joule's loss of the coil is lost by such air blowing.
In an investigation of the present inventors, such lost energy may amount to several tens of watts, and a fact that such heat cannot be effectively utilized for fixation is against energy saving. Also unless an appropriate air blowing method is adopted, even the heat of the heating roller may also be dissipated and lost, thereby leading to a further waste of the energy.
Also among the methods of reducing the heat generation of the coil itself thereby causing a curve of the coil temperature to saturate at a lower temperature, the method of increasing the diameter of the wire thereby reducing the electrical resistance of the coil has a drawback that the electrical resistance is not much lowered, since, in the fixing apparatus of induction heating type, the excitation coil is usually given a high frequency current so that the current flows only in a surface portion of the conductor by so-called “skin effect”.
As a countermeasure therefor, it is conceived to form the coil with a twisted wire (litz wire) formed by twisting plural insulated wires (element wires), but such method results in a higher cost. Also in a configuration employing a twisted wire, since the element wire constituting the twisted wire is insulated, the fixing apparatus has to be so designed that the heat-resistant temperature is not exceeded. As polyimide is usually employed for the insulation of the element wire, it is necessary to design the coil in this case in such a manner that the coil temperature does not exceed 220° C. Stated differently, in case of employing a twisted wire, the upper limit of the temperature of the coil assembly is restricted by the heat resistance of the insulating material of the element wire constituting the twisted wire. Therefore, the fixing apparatus of inducting heating type employing such twisted wire is difficult to apply to the printer with a high output per unit time, in which the electric power supplied to the coil has to be increased.
Also in the method of forming the cylindrical conductive member (heating roller) with a material of a high electrothermal converting efficiency, such cylindrical conductive member is required, in constituting the fixing apparatus, not only to have a high heat generating efficiency but also to meet other conditions such as durability and working property, and it is difficult to meet all these requirements.
Also a composite material formed by laminating aluminum, nickel, copper etc. and called a clad material is utilized for example in a rice cooker etc. utilizing the induction heating method, but such material inevitably becomes expensive.